Foundry core compositions



United States Patent 3,471,429 FOUNDRY CORE COMPOSITIONS Donald Edward Hayford, Hopewell, Va., assignor to Allied Chemical Corporation, New York, N.Y., a corporation of New York No Drawing. Filed Oct. 23, 1965, Ser. No. 504,197 Int. Cl. (308g 13/00; B22c 1/16 U.S. Cl. 260--29.4 2 Claims ABSTRACT OF THE DISCLOSURE and inclusion of copper nitrate further improves cure rate.

This invention relates to foundry core compositions and more particularly to improved foundry core binders and processes for their preparation.

A foundry core may be defined as that part of shaped sand and binding material that aids in forming an internal part of a metal casting. A foundry core should satisfy several requirements. It should usually cure rapidly; it should retain sufficient strength at molten metal temperature to resist erosion by metal flow; it should evolve as little gas as possible during this period and that which does evolve should freely escape; it should have sufficient tensile strength so that it will not crack or distort while being handled, and lastly it should shake out readily after the metal casting has hardened.

To satisfy some of these requirements it has long been known to the foundy art to employ sand along with a suitable binder. Foundry core binders serve to bond the sand or other refractory material which forms the cavity of a casting until the metal solidifies. A binder is initially liquid or powder which is easily mixed with the sand.

A number of binders have been used by the foundry industry. The oldest and still most widely used are those based on drying oils, but these have the disadvantage of requiring long curing times.

Gradually, drying oil types are being modified or replaced by faster baking urea and phenolic resins. Their use in the so-called shell process, by which thin-walled cores or molds are produced by blowing resin-coated sand against a heated pattern, promised to revolutionize foundry core production by eliminating long bake times required by oils and oven-cured resins. Although gaining wide acceptance, the shell process has generally not been able to meet the cost and high production requirements of the foundry industry.

Recently, a new process, the hot box process, was developed to overcome the shortcomings of the shell process. The hot box process also employs a heated pattern box, but instead of a dry, free-flowing resin-coated sand, a wet sand mix is used to produce solid cores. Because of lower resin costs, simpler sand preparation and faster curing, solid cores of small or moderate size made by the hot box process are less expensive than shell process cores.

Most hot box binders contain urea-formaldehyde in one form or other. However, because urea resins not only cure rapidly but also overcure rapidly, the hot box binders composed thereof are modified by more thermal resistant resins, such as for example furfuryl alcohol or phenolformaldehyde resins. Additionally, furfuryl alcohol serves 3,471,429 Patented Oct. 7, 1969 "ice another purpose, it gives greater strength cores at molten metal temperatures.

Both furfuryl and phenolic modified binders require an acidic or acid-releasing catalyst to obtain the maximum rate of cure in the hot box. In the usual commercial practice, catalyst and binder are added separately to core sand and mulled until sand, binder and catalyst are intimately mixed. This sand mix is transported to storage hoppers located over the core blowers. From the hoppers the sand is fed to core blowers which expel the sand mix into the heated cavities of the hot box. After a cure time varying with box temperature, core size, binder system and sand type, the hot boxes are opened and the hot cores ejected. After a brief cooling period, during which the binder continues to cure, the cores are generally dip coated or sprayed with a slurry of a fine refractory material to give a smoother, more dense surface against molten metal. After passing through a drying oven, the cores are ready for mold assembly and metal pouring.

The use of furfuryl alcohol in known hot box processes as exemplified for example by US. 3,168,489 to Brown and Watson, Feb. 2, 1965, solves one problem while it creates another. As mentioned, furfuryl alcohol is employed by the art as a cure inhibitor. Obviously, an excess of furfuryl alcohol will slow the cure rate to the point where it is impracticable for use in foundry core production. Yet an excess of furfuryl alcohol is highly desirable since it gives greater strength cores at molten metal temperatures.

It is therefore an object of this invention to produce a foundry core that has high tensile strength and high hot strength.

It is another object of this invention to produce a foundry core composition that will cure rapidly while retaining its high tensile strength and high hot strength.

It is still another object of this invention to produce a foundry core binder system that will impart the abovementioned objects to a foundry core.

While it was known to use furfuryl alcohol as a core inhibitor in hot binders, and to im art tensile strength to foundry cores produced therefrom, the seemingly impossible problem was how to employ high amounts of furfuryl alcohol without impeding cure rates. In the inventors search for a solution it was though that greater amounts of catalyst could be utilized to overcome the cure retarding effect of increased furfuryl alcohol. However, in order to introduce more catalyst, more water was needed to dissolve the catalyst, and the increased Water, in turn, slowed the cure rate in the hot box. It was then discovered that approximately equimolar mixtures of urea and ammonium salts selected from ammonium nitrate and ammonia thiocyanate are highly soluble in water, and therefore more catalyst may be employed without increasing the water content in the process. Surprisingly, mixtures of urea and these salts in approximately equimolar amounts are appreciably more soluble in water than either taken alone.

In accordance with this invention a binder system for admixture with foundry sand in the hot box process is prepared which consists of a binder component comprising between about 40% and about 60%, preferably between about 45% and 55%, by weight of furfuryl alconol, the balance aqueous urea-formaldehyde concentrate, and an acid catalyst component comprising between about 40% and about 55%, preferably between about 45% and 50%, by weight of an ammonium salt selected from the group consisting of ammonium nitrate and ammonium thiocyanate, and between about 30% and about 45%, preferably 35% to 40%, by weight urea, the balance water.

In one preferred embodiment of this invention the binder component comprises an essentially equal weight mixture of furfuryl alcohol and liquid urea-formaldethyde concentrate, an the acidic catalyst component comprises equimolar proportions of ammonium nitrate and urea dissolved in the minimum amount of water needed for solution.

For purposes of this invention, readily available commercial furfuryl alcohol is satisfactory. The liquid ureaformaldehyde concentrate useful in this invention should be miscible with and storable with furfuryl alcohol. It should contain not more than about 20% by weight of water, preferably not more than and it should also contain between about and about 30% by weight of free formaldehyde, available to react with the ammonium salt to obtain high acidity during cure. A ureaformaldehyde having these desirable properties is produce in accordance with US. 2,652,377 to Kise, Sept. 15, 1953, and is available commercially as U.F. Concentrate 85 from Allied Commercial Corporation, New York, N.Y. It contains about 60% free an combined formaldehyde, about urea, and about 15% water,

all by weight. The mol ratio of total formaldehyde to urea in the concentrate is not critical except that for stability reasons it is preferably in the range set forth in U.S. 2,652,377, above. Both the concentrate and furfuryl alcohol should be close to neutral, i.e., about pH 5 to 9, and free of large quantities of buffering agents.

For the catalyst component ordinary commercially available urea and ammonium nitrate are satisfactory. While ammonium thiocyanate has been found to be quite soluble in water in equimolar mixture with urea, it is slightly more expensive than ammonium nitrate, and for this reason ammonium nitrate may be preferred. Many other ammonium salts were tried and it was found that no salts, other than ammonium nitrate and ammonium thiocyanate, are effective in this invention.

A foundry core composition is prepared by admixing the subject binder and catalyst components with sand. Binder component is added in an amount between about 1 and 3% by weight of sand; catalyst component is added in amount between about 20% and 40%, preferably between 25% and 35%, by weight of the binder component.

The amount of binder component required will vary with the size and configuration of the core, type and fineness of the sand, and with the molten metal being poured. Large, simple cores may be made with low binder content, whereas cores with fine projection require more strength and a higher binder content. Generally, the smaller the sand particle size the higher the binder content required to obtain a desired degree of strength, however lake sands, containing fractional percentages of clay, require a higher binder content than do clean silica sands of the same fineness. For most conditions, the binder content, as mentioned above, may be as low as between about 1 and 3%. While higher amounts may be used, it is considered unnecessary and wasteful since the amounts as set forth herein produce high strength foundry cores. Except for convenience in sand preparation, there is no requirement that the concentrate and furfuryl alcohol must be premixed. The two binder ingredients may be added separately to the sand in proper proportion.

When small cores are being blown, cure speed is less critical but sand storage life may become a problem because the sand mix is being consumed more slowly. On the other hand, when large cores are being blown, cure speed is important and sand storage life is less critical. Catalytic adjustment may be necessary to compensate for seasonal temperatures or for slightly alkaline or acidic sands. For example, to realize longer sand storage life or for use with acidic sands, the subject catalyst component may be modified by the inclusion of up to about 4% by weight of ammonia. For faster curing, or for use with alkaline sands, up to about 5% by weight of copper nitrate may be added. The buffering agents need only have the requirement that they be compatible with the high concentration of urea and ammonium salt in the catalyst. In addition to ammonia, sodium hydroxide and potassium hydroxide are good alkaline buffers. Ferrous nitrate, aluminum nitrate and chromic nitrate are illustrative of additional acidic buffers. Gentle heating will aid in the preparation of the catalyst solution, however, they may be prepared by stirring at room temperature.

In preparing the sand mix, the catalyst component is added separately to the sand, apart from the binder component, however either binder or catalyst may be added first, except when alkaline buffer is used in the catalyst, then preferably the binder is added first. Since both binder and catalyst components are non-viscous, long mulling times are not required, and as little as one to three minutes is sufficient after each addition. Hot box temperatures should be in the 350 to 500 F. range used for conventional processes. Required cure time will vary greatly with core shape and size, among other factors, but will usually be in the range of 10 to 50 seconds.

While the exact mechanism by which this invention works is not known, it is believed to be grounded in the following principles. The high furfuryl alcohol content of the binder component, required for high strength foundry cores, would seriously retard cure rate were it not for the high free formaldehyde content in conjunction with the high concentration of ammonium nitrate (or thiocyanate) in the catalyst component. This high, free formaldehyde would produce a fume problem were it not for the high concentration of ammonium salt which reacts with it in turn. The high ammonium nitrate content would be impossible except for the high solubility of ammonium nitrate and urea in water. Finally, the high concentration of the acid-producing reactants, i.e. ammonium nitrate and free formaldehyde, would result in overcure were it not for the high furfuryl alcohol content of the binder component.

The invention will be described further in conjunction with the following specific examples, but it is to be understood that these are merely for the purposes of illustration and are not intended to limit the invention thereto.

Example 1 This example illustrated the superior hot strength and high tensile strength of the invention binder system over commercial hot box binders. Three sand mixes were prepared using a clean silica sand of 65 AFS (American Foundry Society) fineness. Each mix contained 2% binder based on weight of sand. For the first mix, a commercial furfurylated hot box binder and catalyst were used. For the second a commercial phenolic hot box binder and catalyst were used. Both of these binders were of the type recommended for ferrous castings. In preparing these mixes, the manufacturers recommendations as to catalyst concentrations and mulling times were followed. For the third mix, a binder was prepared by mixing equal weight quantities of furfuryl alcohol and UF Concentrate85. A catalyst for this binder was prepared by dissolving, with heat, 40 grams ammonium nitrate and 40 grams urea in 10 grams water (44.5% NH NO 44.5% urea, 11% H O). This catalyst has a dissolution temperature slightly above room temperature, but can be stored for brief periods at room temperature; 34% catalyst based on weight of binder was used. To evaluate cure rates and cold tensile strengths, standard l-inch AFS tensile briquettes were blown with a Redford HP-4 core blower using a box temperature or 450:5" F. and cure times of 10, 20 and 30 seconds. Three tensile briquettes were blown from each mix at each cure time. Approximately, one hour after blowing, the tensile briquettes were tested on a Dillon Model LW Universal Tester set on 0.5 inch per minute travel. Strengths which differed from the mean by more than 15 were discounted. One-inch cubes were cut from the 20-secoud briquettes after testing for tensile strengths. The cubes were placed in a 1000" C. muiile furnace for 30 seconds and then immediately tested for compressive strength. One purpose of this test was to determine how much bond strength remained after a brief exposure to temperatures approaching that of molten metal -(grey iron has a melting point of about 1230 0.). Results are given in Table 1 below.

TABLE 1.HOT COMPRESSIVE STRENGTH AND COLD TENSILE STRENGTH Average hot Cold tensile compressive Cure time, strength, strength, Binder sec. p.s.i. p.s.i.

Commercial turiurylated..- 285 Not tested 20 373 I 145 30 380 Not tested Commercial phenolic 10 303 Not tested 20 332 138 30 457 Not tested 50% furfuryl alcohol 10 508 Not tested 50% UF concentrate-85 20 47B 475 30 525 Not tested Example 2 This example showed the rapid strength build-up of the invention binder system compared to commercial hot box binders. Two sand mixes were prepared using a clean silica sand of 65 AFS fineness. The first mix was made with a commercial furfurylated hot box binder, 2% binder based on weight of sand, and the recommended catalyst, 20% based on weight of binder. For the second mix, the binder was an equal weight mixture of furfuryl alcohol and UP Concentrate-85, 2% binder based on weight of sand, and the catalyst was a solution of 45% by weight ammonium nitrate and 34% urea in 21% water, used 22% based on weight of binder. Standard l-inch AFS tensile briquettes were blown as in Example 1 using a cure temperature of 425 :5 F. Tensile strengths were determined after the briquettes were removed from the hot box. Three tests were made at each cure and cooling time and the results averaged. Results are given in Table 2 below.

TABLE 2.WARM STRENGTH In this example, the invention binder system was compared with commercial hot box binders in resistance to molten metal. Three sand mixes were prepared, each contaning 2% binder based on weight of sand and 20% catalyst based on weight of binder. The first mix was prepared with a commercial furfurylated hot box binder and catalyst, the second mix was prepared with a commercial phenolic hot box binder and catalyst and the third mix was prepared with a binder consisting of 52% furfuryl alcohol and 48% UP Concentrate-85 and a catalyst consising of 48% ammonium nitrate, 36% urea, 1.1% ammonia and 14.9% water. Both the commercial furfurylated and phenolic binders were of the type recommended for ferrous castings. Standard l-inch AFS tensile briquettes were made from each mix using a cure time of 20 seconds and 450 F. box temperature. The cured briquettes were pasted to the bottom section of a baked sand mold so that the top and sides of each briquette would be exposed to molten metal. The mold was filled with grey iron at approximately 2500 F.

After cooling, the casting was removed from the sand and inspected. Results are given in Table 3 below.

TABLE 3.RESISTANCE TO MOLTEN METAL [Grey iron at 2,500 F.]

Etiect on core cavity 20 Burn-in, veinlng. 20 Veining. 20 No burn-in, no veining.

l Occlusion of sand grains in the metal surface caused by premature binder breakdown.

I Thin, parallel projections of metal resulting from the cracking of the core before the metal solidifies.

Example 4 This example showed the effect of binder composition on cure rate and sand storage life. Three binders were prepared by mixing furfuryl alcohol and UP Concentratein 55/45, 50/50 and 45/55 weight ratios. A catalyst was prepared by dissolving 48 grams ammonium nitrate and 36 grams urea in 16 grams water. Three sand mixes were made using the above binders and catalyst and a blend of 85% lake sand of 45 AFS fineness and 15% bank sand of 97 AFS fineness. For each of the three sand mixes, 2000 grams of the sand blend was charged to a 12-inch Cincinnati muller, 8 grams of catalyst was added, and the mixture was mulled 2 minutes; then 40 grams binder was added, and mulling was continued another 2 minutes. Proportions were 2% binder based on weight of sand and 20% catalyst based on weight of binder. To evaluate sand storage life, standard 2-inch AFS green strength cylinders were rammed from each sand mix and stored in sealed containers for 3 hours at 100 F. At the end of this period, compressive strengths of the cylinders were determined. By this procedure a low compressive trength indicates good sand storage life and a high compressive strength means poor sand storage life. Results of these tests are given in Table 1. To evaluate cure rates and cold tensile strengths, standard l-inch AFS tensile briquettes were blown with a Redford HP-4 core blower using a box temperature of 350:5" C. and cure times of 10, 20 and 30 seconds. Three tensile briquettes were blown from each mix at each cure time. Approximately, one hour after blowing, the tensile briquettes were tested on a Dillon Model LW Universal Tester set on 0.5 inch per minute travel. Strengths which diiiered from the means by more than 15% were discounted. Results of these tests are given in Table 4 below. Overall tensile strengths in the table were low because of clay present in the bank said. However, the tendency of the binder containing 45% furfuryl alcohol to overcure was shown by the decline in tensile strength with cure time.

TABLE 4.EFFECT OF BINDER COMPOSITION This example showed how cure rate can be improved by the addition of an acidic buffer to the catalyst solution. Two sand mixes were prepared as in Example 4 using an alkaline lake sand of 50 AFS fineness. Both mixes contained 2% binder based on weight of sand and 20% catalyst based on weight of binder. A 50% by weight furfuryl alcohol, 50% UP Concentrate-85 binder was used for both mixes. The catalyst for the first mix consisted of 48% by weight ammonium nitrate and 36% urea in 16% water. For the second the catalyst consisted of 48% by weight ammonium nitrate, 35% urea and 3% Cu(NO -3H O in 14% water. Standard l-inch AFS tensile briquettes were blown and tested as in Example 4, using a cure temperature of 350 F. and 10, 20 and 30 second cure times. Three briquettes were made from each mix at each cure time. Results are given in Table below. Compared with results with nonbuffered catalyst, the higher strengths obtained at and second cure time with the buffered catalyst are indicative of faster curing.

TABLE 5.-ACIDIC BUFFERS Example 6 This example illustrated how sand storage life may be improved by the addition of an alkaline buffer to the catalyst solution. A binder was prepared by mixing equal weight quantities of furfuryl alcohol and UP Concentrate-85. Two catalysts were prepared. Both contained 50% by weight ammonium nitrate and 37.5% urea but the second was prepared using 28% aqueous ammonia as solvent instead of water to give a catalyst containing approximately 3.6% by weight ammonia. Sand mixes were prepared by a procedure similar to that in Example 4 except binder was added first. Two percent binder based on weight of sand and 20% catalyst based on weight of binder were used on a lake sand of 46 AFS fineness. Standard l-inch AFS tensile briquettes were blown and tested as in Example 4, using a box temperature of 350i5 F. and cure times of 10 and 30 seconds. Green strength cylinders were rammed from each mix and 4 stored in sealed containers for 3 hours at 100 F. and then tested for compressive strength. Test results are given in Table 6 below. Table 6 shows that sand storage life may be improved by the use of an alkaline buffer.

TABLE 6.ALKALINE BUFFERS Compressive Average cold strength after Cure time, tensile strength, 3 hours at Catalyst sec. p.s.i. 100 F., p.s.i. 5 N0 NHa 10 235 1. 75

10 I cla1m.

1. A curable foundry core composition comprising in admixture a binder component, an acidic catalyst component, and sand: said binder present in amount between about 1 and about 3% by weight of said sand and containing between about 40 and about 60% by weight of furfuryl alcohol, the balance aqueous urea-formaldehyde concentrate having not more than about 20% by weight of water; said acidic catalyst present in amount between about 20 and about 40% by weight of said binder and comprising between about 40 and about 55% by weight of ammonium nitrate, between about 1 and about 5% by weight of Cu(NO -3H O and between about 30 and by weight of urea, the balance water.

2. A foundry core binder system comprising a binder component containing between about 40 and about 60% by weight of furfuryl alcohol, the balance aqueous ureaformaldehyde concentrate having not more than about 20% by weight of water, and an acidic catalyst component comprising between about 40 and about by weight of ammonium nitrate, between about 1 and about 5% by weight of Cu(NO -3H O, and between about 30 and 45% by weight of urea, the balance Water.

References Cited UNITED STATES PATENTS 2,236,184 3/1941 Menger 260-71 3,059,297 10/1962 Dunn et al. 260-71 3,100,754 8/1963 Booth et a1. 260-69 3,182,030 5/1965 Parkes.

3,247,556 4/1966 Buell et a1. 260-29.4 3,297,611 1/1967 Hill 26029.4 3,360,492 12/1967 Tsou 260---29.4

5 MURRAY TILLMAN, Primary Examiner J. C. BLEUTGE, Assistant Examiner U.S. Cl. X.R. 

